1. Field of the Invention
This invention relates to a moisture sensor and a method for the production of the same. More particularly, it relates to a technique for the formation of electrodes of a moisture sensor.
2. Description of the Prior Art
Moisture sensors having a variety of moisture sensitive materials such as metal oxides, polymer materials, etc., have been developed as sensor means for detecting humidity and have the possibility of an enlarged range of applications with regard to air-conditioning machinery, medical treatment apparatus, automobiles, agriculture and forestry machinery, etc. These moisture sensors are roughly classified into two groups, one of which is a resistance-variation format including moisture sensors having a moisture sensitive material such as Fe.sub.2 O.sub.3, SnO.sub.2, LiCl, etc. This resistance-variation format is based on the fact that the electroconductive properties of various ions, protons, electrons, etc., in the moisture sensitive material vary depending upon a variation in water-content therein. The other group is of an electrostatic capacitance-variation format including moisture sensors having a moisture sensitive material such as Al.sub.2 O.sub.3, cellulose, etc. This electrostatic capacitance-variation format is based on a characteristic that the dielectric constant of the moisture sensitive material varies depending upon a variation in the water-content therein resulting in a variation of the electrostatic capacitance. There have been exceptional moisture sensors, having a moisture sensitive material such as certain polymers (e.g., polyvinylalcohol), which function as either an electrostatic capacitance-variation format sensor or a resistance-variation format sensor depending upon the range of humidity to be detected.
Electrostatic capacitance-variation type moisture sensors are advantageous over resistance-variation type moisture sensors in that the moisture sensitive characteristic of the electrostatic capacitance-variation type sensors varies linearly with relative humidity and exhibits almost no variation with changes in temperature.
In order to utilize the characteristics of the above-mentioned various kinds of moisture sensors, an electrode structure which is suitable for the characteristic of each of these sensors must be designed. To the conventional moisture sensors having a moisture sensitive material in a thin film formed on an insulating substrate, the following two kinds of electrode structure have been applied:
One of the two kinds of electrode structure is a comb-shaped structure comprising a pair of comb-shaped electrodes and a moisture sensitive film on the comb-shaped electrodes to electrically connect the comb-shaped electrodes with each other. The electrodes are disposed on a substrate by a vacuum evaporation technique or a screen printing method in a manner that each of the protrusions of one of the comb-shaped electrodes faces the concave surface of the other. Such a comb-shaped structure is generally applied to resistance-variation type moisture sensors, the electrodes of which are formed before the formation of the moisture sensitive film so that the desired material for electrodes having an excellent adherence to the substrate can be selected.
The other electrode structure is a triple-layered structure which comprises an upper electrode, a bottom electrode and a moisture sensitive film sandwiched between these electrodes, resulting in a large electrostatic capacitance. Such a triple-layered structure is generally used for electrostatic capacitance-variation type moisture sensors, in which an extremely thin film of a noble metal such as Au, Pt, etc., is used as the upper electrode which must have moisture permeability and corrosion resistance. Such a noble metal thin film is inferior in adherence to the substrate thereby making difficult the drawing of a lead wire therefrom. When a lead wire is connected to the thin noble metal film by means of an Ag paste, etc., the area of the noble metal film sticks to the substrate which corresponds to the connecting portion of the noble metal film, and the lead wire becomes weak causing ready separation of the noble metal film from the substrate and/or breaking thereof. In order to eliminate such problems in actual use, the following approaches for attaching the noble metal film to the substrate have been proposed:
(1) an approach in which the noble metal film is pressed to the substrate by a metal board or a metal ring; PA1 (2) an approach in which the moisture sensitive film is formed on the bottom electrode divided into two parts and the extremely thin electrode film which is moisture permeable is formed on the moisture sensitive film, and then the lead wires, respectively, are connected to the two parts of the bottom electrode; and PA1 (3) an approach in which a portion of the upper electrode is extended to a connection terminal on a substrate on the outside of the moisture sensitive film having a pattern and a lead wire is drawn from the connection terminal. PA1 (1) disposing a metal film on a substrate, PA1 (2) subjecting said metal film to a patterning treatment to form a bottom electrode and a pair of connection terminals, one of which is extended from the bottom electrode and the other of which is to be connected to an upper electrode, PA1 (3) disposing a moisture sensitive film on said bottom electrode, PA1 (4) disposing said upper electrode on said moisture sensitive film to connect a step-shaped extension protruding from said upper electrode to said connection terminal on the substrate for the upper electrode, and PA1 (5) covering said step-shaped extension with a metal film.
The first approach mentioned in Item (1) has a problem in that the electrode structure is complicated so that the electrodes tend to be damaged in the assembly process. The second approach mentioned in the Item (2), although having no problems in the production process, is not compact due to reduced moisture sensitivity resulting from such an electrode structure. This is because the equivalent network thereof indicates that, as compared with an equivalent network of a different electrode structure having a non-divided bottom electrode on a substrate, two halves of the electrostatic capacitance (C/2) of the moisture sensitive film are connected to each other in series resulting in a one fourth electrostatic capacitance of the moisture sensitive film (represented by the formula: ##EQU1## per effective area of the substrate produced by the equivalent network indicated by the above-mentioned different electrode structure.
The third approach mentioned in the Item (3) has the problems that adherence between the substrate and the noble metal thin film is weak in the case where the surface of the substrate is smooth, such as a glass substrate, a Si substrate coated with SiO.sub.2, Si.sub.3 N.sub.4, etc. and the like and the step portions in the pattern of the moisture sensitive film tend to be broken.